Spurious signal correction for surface acoustic wave (SAW) security devices

ABSTRACT

The invention features a theft resistant security system for motor vehicles. The system has receptacle with a coupling coil. A SAW (surface acoustical wave) device associated with the receptacle for relative movement thereto is also provided. The SAW device has a code and its own coupling coil for interaction with the receptacle coupling coil. A testing mechanism is connected to the receptacle for determining whether the code of the SAW device is valid for operation of the motor vehicle. Problems of crosstalk and wave reflection may produce spurious code pulses in systems of this type. The invention provides techniques for eliminating or reducing these unwanted pulses.

RELATED APPLICATION

This application is related to co-pending patent application Ser. No.227,282; filed Aug. 2, 1988, titled CODED SURFACE ACOUSTICAL WAVE (SAW)MOTOR VEHICLE SECURITY DEVICE and is meant to incorporate by way ofreference all of the teachings and description therein.

1. Field of the Invention

The invention relates to automotive antitheft devices utilizing surfaceacoustic wave (SAW) devices, and more particularly to automotiveantitheft SAW devices having corrective features for eliminating and/orreducing spurious signal detection.

2. BACKGROUND OF THE INVENTION

The invention pertains to a surface acoustic wave (SAW) device whichforms part of an antitheft automotive system, as described in theaforementioned patent application, Ser. No. 227,282. The previousapplication describes a SAW device comprising a coded SAW delay line andattached coil antenna that is embedded in or integrally formed with anotherwise conventional mechanical ignition key.

A coil mounted around the lock cylinder mounted in the steering columnprovides a brief radio frequency pulse. This pulse is picked up by thecoil antenna, which excites a surface acoustical wave on the SAW delayline. After a time delay, the antenna is re-excited by a coded set ofpulses determined by the arrangement of interdigital transducers (IDT)on the SAW delay line. These transponded pulses are picked up by theoriginal coil, and processed electronically to ensure that the code onthe SAW delay line matches a predetermined code. If there is no correctmatch, this is interpreted as an unauthorized intrusion and furtheraction is inhibited.

A major problem that is encountered with SAW devices of this type is theexistence of spurious signals. In the present context, the term spurioussignal means unintended transponded signals that interfere with thecorrect code interpretation. These spurious signals arise from twosources: (1) crosstalk; and (2) double transit interference.

Crosstalk refers to output pulses created by launching and receivingSAWs within the subIDT structure, e.g., a SAW launched by a subIDT andreceived by another subIDT. Double transit interference refers to outputpulses created by two SAW transits. For example, a SAW launched by themain IDT and received by a subIDT contributes to an output pulse; thisoutput pulse excites the coil antenna as desired. However, acousticenergy is also reflected from a subIDT, and redirected and picked up byother subIDTs as well as the main IDT.

An initially developed coded SAW delay line based system for electroniclabeling provided a means to reduce crosstalk spurious pulses byarranging the SAW delay line so that the SAW transit time from the mainIDT to the first subIDT exceeded the total transit time from the firstsubIDT to the last subIDT.¹ In this way, all of the crosstalk spuriouspulses arrived before the main, wanted, code-bearing pulses. It wasrecognized that it was important to provide additional means forsuppressing spurious signals.² Means were devised using parallelacoustic wave paths to achieve this suppression.

Other means included the use of phase coding. The utilization ofmicrowave frequency and decoding of a phase coded signal, however,requires complex and expensive electronics.

In an automotive antitheft system it is most important to reduce coststo a minimum. To realize this objective, the rf frequency should bebelow 50 MHz, so that low cost electronic components, currentlyavailable for other consumer devices, can be employed.

However, when the rf frequency is reduced below 50 MHz, spurious signalscannot be eliminated effectively by utilizing either parallel acousticwave paths or phase coding techniques.

The restrictions on frequency, chip size and coding method (amplitudeencoding) imposed by the automotive antitheft system with spurioussignal elimination at the lower frequency leads to an unreasonably lownumber of available codes.

The present invention relaxes the previous requirement that the SAWdevice transit time from the main IDT to the first tap subIDT exceed thetransit time from first subIDT to the last subIDT, thus increasing thenumber of available code bits. This provides a greater number of codesfor an effective automotive antitheft system, but requires a newtechnique for eliminating or reducing spurious signals.

The current invention uses the following new techniques to eliminatespurious signals:

(1) time narrowing;

(2) time shifting;

(3) time spreading; and

(4) cancellation.

Time narrowing uses narrower subIDTs with respect to the main IDT, thusdiminishing crosstalk signals.

Time shifting utilizes the addition of a half code period to the spacingbetween the main IDT and the subIDTs, thus shifting the crosstalksignals between recognized signal centers.

Time spreading operates to shape the pattern of electrode overlapswithin the subIDTs in order to spread crosstalk signals over a longerperiod of time, thus weakening these spurious signals.

Cancellation of the crosstalk signals is accomplished by addingadditional IDTs, which generate additional signals of equal amplitudeand time, and opposite phase, relative to the crosstalk signals.

Also contemplated by the present invention is the reduction orelimination of reflected acoustic energy resulting in double transitinterference. This is accomplished by connecting an external resistoracross the SAW device coil or by terminating the detection coil with aresistor.

Also additional IDTs can be connected to the bus bars which do notinteract acoustically with the code-generating IDTs by the utilizationof an acoustic absorber.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided a theftresistant security system for motor vehicles. The system has areceptacle with a coupling coil. A SAW device associated with thereceptacle for relative movement therein is also provided. The SAWdevice has a code and its own coupling coil for interaction with thereceptacle coupling coil. A testing mechanism is connected to thereceptacle for determining whether the code of the SAW device is validfor operation of the motor vehicle.

The SAW device comprises a substrate having a plurality of taptransducers serially disposed thereon. The tap transducers arespaced-apart from each other, and each tap transducer position defines abit of the code.

A first tap transducer in the series is disposed no transducer. Each taptransducer is operatively connected to the launch transducer by a pairof bus bars. The coupling coil is connected to the bus bars.

In order to reduce crosstalk, at least one, but preferably all of thetap transducers have a relatively narrower electrode width than theleading transducer.

Also, the first tap transducer in the series can be disposed afractional part of a bit length further than a given length of bits fromthe leading transducer. This displaces as well as weakens any crosstalkpulse signal.

Another way of reducing crosstalk is provided by designing an electrodegap pattern of one transducer opposite to the direction of the electrodegap patterns of the other two transducers.

Yet another method of reducing crosstalk involves phasing at least onetap transducer oppositely to a crosstalk signal pulse of equalamplitude.

Double wave transit interference is reduced by increasing the loadconductance. This is accomplished by adding resistance to eithercode-production or detection coupling coils.

Also, tap transducers that do not interact with the other tap transducercan be utilized for the purpose. Such tap transducers can include anacoustic absorber.

BRIEF DESCRIPTION OF THE DRAWINGS

A complete understanding of the present invention may be obtained byreference to the accompanying drawings, when taken in conjunction withthe detailed description thereof and in which:

FIG. 1 is a perspective view of a steering column, steering wheel andcomputer processor;

FIG. 2 is an exploded perspective assembly view of the preferredembodiment of the present invention;

FIG. 3 is a cross sectional view of a key inserted in an ignitionswitch;

FIG. 4 is a schematic representation of a key with a SAW device insertedin proximity to a coupling coil;

FIG. 5 is a schematic representation of a key with a SAW device imbeddedtherein;

FIG. 6 is a schematic representation of a SAW device shown connected toa coupling coil;

FIG. 7 is an enlarged plan view of a typical coded SAW device showingthe electrode patterns;

FIG. 8 is a diagrammatic representation of the electrical pulse signalproduced by the coded SAW device of FIG. 7;

FIG. 9 is enlarged plan view of the coded SAW device of FIG. 6 having anarrowed electrode pattern;

FIG. 10 is a diagrammatic representation of the electrical pulse signalproduced by the coded SAW device of FIG. 9;

FIG. 11 is an enlarged plan view of the coded SAW device of FIG. 9having the first tap transducer electrode displaced an additional bitlength fraction from the launch transducer electrode;

FIG. 12 is a diagrammatic representation of the electrical pulse signalproduced by the coded SAW device of FIG. 11;

FIG. 13 is an enlarged plan view of a coded SAW device with taptransducer electrodes having patterns shaped in opposite directions withrespect to each other;

FIG. 14 is a diagrammatic representation of the electrical pulse signalproduced by the coded SAW device of FIG. 13;

FIG. 15 is an enlarged plan view of a coded SAW device similar to thatshown in FIG. 7, wherein an additional tap transducer has been added tobit position 3 in order to cancel the crosstalk pulse; and

FIG. 16 is a diagrammatic representation of the electrical pulse signalproduced by the coded SAW device of FIG. 15.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Generally speaking, the invention features a coded SAW device forautomotive antitheft systems that is corrected for crosstalk andreflective signals. The device and detection circuitry is described inaforementioned U.S. patent application, Ser. No. 227,282.

For purposes of brevity, the teachings and description of the aforesaidapplication are incorporated herein by reference.

Referring now to FIG. 1, there are shown a conventional steering wheel10 and steering column 12, as can be found in most motor vehicles andespecially in automobiles. Connected to an ignition switch 14,hereinbelow described in greater detail, is a microprocessor 16. Manymotor vehicles have electronic brains or computer processors such asthat shown as reference numeral 16, used to regulate the electrical,mechanical and chemical systems used in the vehicles. Often, generalpurpose computers, a network thereof, or microprocessor-based electronicsystems are used for the various functions. For example, a computerprocessor in an automobile may be used to regulate gas flow, to signalmalfunctions in brake systems, to indicate the level of oil in thecrankcase, to adjust internal temperature and the like.

A collar housing 18 is shown mounted on the steering column 12 by meansof screws 20, although any suitable mounting means may be used for thispurpose. The collar 18 houses a coupling coil or antenna, not shown, theuse of which is explained hereafter. Certain circuitry may also becontained within the collar 18.

Referring now also to FIG. 2, there is shown a perspective assembly viewof the preferred embodiment of the present invention. The collar housing18 has a circular aperture 19 cut therein. A printed circuit board orcard 22 is housed by the collar housing 18 and encircles the ignitionswitch, not shown. The printed circuit board 22 has components mountedthereon that function as a receiver in cooperation with a SAW device ashereinbelow described. Encircling the printed circuit board aperture isa coupling coil 24, the use of which is described in greater detailhereinbelow.

An ignition key is shown generally at reference numeral 26. The key 26is adapted to be inserted through the collar aperture 19 and into theignition switch, not shown. It should be noted that a conventionalignition key with a coded mechanical blade is not necessarily arequirement of the present invention.

Referring now also to FIG. 3, there is shown a cross sectional side viewof an ignition key 26 inserted into the ignition switch 14.

Referring now also to FIG. 4, there is shown a schematic representationof an ignition key 26, the outline of which is shown in phantom. In thepreferred embodiment, the key 26 has a longitudinal mechanicalprotuberance 28, as do most conventional ignition keys. Thislongitudinal section 28 is preferably fabricated of metal and ismechanically coded to fit a particular ignition key lock, not shown.

Connected to the metal protuberance 28 is a key handle 30. The handle 30has an aperture 32 adapted to fit onto conventional key holders and keyrings, not shown. The handle 30 can be rubber, plastic or any othersuitable nonmetallic material. Plastic is preferred. Imbedded in thehandle 30 is a surface acoustical wave (SAW) device 34. Connected to theSAW device 34 is a coupling coil or antenna 36. The coil 36 forms acontinuous loop connected to the SAW device 34 at ports 38 and 40 and isdisposed perpendicular to the major axis of the key 26 and the SAWdevice 34.

Also shown in FIG. 4 is a second coupling coil or antenna 42, whichencircles the SAW device coil 36, but is not connected thereto. Thesecond coil 42 is connected to a sensor, not shown, by means of anelectrically conductive cable 44. It can be seen that the key 26 and SAWdevice coil 36 are adapted to move relative to the second coil 42.

Referring now also to FIG. 5, there is shown another cross sectionalview of an ignition key in accordance with the present invention. Inthis embodiment, the SAW device coil 36 is disposed parallel to themajor plane of the key 26 and of the SAW device 34. This coilconfiguration can also be used with appropriate modification to thesensor coil 42 (FIG. 4).

Referring now also to FIG. 6, there is shown a representation of a SAWtransponder, which is imbedded in an ignition key 26 and to which isconnected a coupling coil 36. It should be noted that other coil orantenna configurations, such as dipole antennas, can also be used.

The transponder operates to convert a received signal to an acousticwave and then to reconvert the acoustic energy back into an electricalsignal for transmission via the coupling coil 36. The signaltransforming element of the transponder includes a substrate ofpiezoelectric material, not shown, on one surface of which is depositeda layer of metal, such as aluminum, forming a 6-bit spatial pattern ofelectrodes or transducers shown in FIG. 6. In alternate embodiments,binary codes of more or less than six bits can be used. Moreover, thecode itself need not necessarily be binary.

The piezoelectric substrate, not shown, is fabricated from YZ lithiumniobate (LiNbO3). Other materials can be used for the piezoelectricsubstrate, such as PZT ceramic and PVDF polymers.

The aforementioned transducer pattern comprises two bus bars 62 and 64connected to the coupling coil 36. A leading transducer 50 and aplurality of coding elements or tap transducers 52, 54, 56, 58, 60, 61are also provided. These transducers are also known as interdigitatedelectrode arrays. The bus bars 62 and 64 define a path of travel, shownby arrow 66, for an acoustic wave which is generated by the leadingtransducer 50 and propagates substantially linearly, reaching the taptransducers 52-61, each in turn. The tap transducers 52-61 convert theacoustic wave back into electrical energy which is collected andtherefore summed by the bus bars 62 and 64. This electrical energy thenactivates the coupling coil 36 and is converted into electromagneticradiation for transmission.

In the preferred embodiment, the tap transducers 52-61 are provided atequally spaced intervals along the acoustic wave path 66. Aninformational code associated with the transponder can be imparted byremoving a selected number of tap transducers 52-61. In alternateembodiments, delay pads, not shown, can be provided between taptransducers 52-61. They can be made of the same material as, anddeposited with, the bus bars 62 and 64 and the tap transducers 52-61,each delay pad having a width sufficient to delay the propagation of theacoustic wave from one tap transducer 52, for example, to the next 54.

A major problem that is encountered with the SAW device described aboveis the existence of spurious signals. In the present context, the termspurious signal means unintended transponded signals that interfere withthe correct interpretation. These spurious signals arise from twosources: (1) crosstalk and (2) double transit interference.

Referring to FIG. 6, a simple explanation for the intended operation ofthe SAW delay line is to imagine that an electrical excitation launchesa SAW from main IDT 50. This wave 66 travels towards a set of subIDTs52, 54, 56, 58, 60 and 61, each of which converts part of the SAW energyback to electrical energy. However, since all IDTs are interconnected,all IDTs both launch and receive SAWs. Crosstalk refers to output pulsescreated by launching and receiving SAWs within the subIDT structure,e.g., a SAW launched by subIDT 52 and received by subIDT 60.

Double transit interference refers to output pulses created by two SAWtransits. For example, a SAW launched by main IDT 50 and received bysubIDT 58 contributes to an output pulse; this output pulse excites thecoil antenna 36 as desired. However, acoustic energy is also reflectedfrom subIDT 54, and redirected and picked up by subIDTs 56, 54 and 52 aswell as the main IDT 50. One may trace through a plethora of similarprocesses.

Cost and range considerations mandate the use of a reduced rf frequency,preferably below 50 MHz so that low cost electronic components developedfor consumer electronics can be employed. Cost considerations alsodictate reducing the number of electronic components, for example, phasedetectors. This mandates amplitude encoding for the SAW device, forexample using the presence of a pulse to indicate a binary 1 and itsabsence to indicate a binary 0. Such a coding scheme simplifies thedetection and decoding electronics but introduces a higher vulnerabilityto spurious signals. Two factors, described below, limit the number ofcode bits available in a low-frequency amplitude encoded system.

(1) Low frequency increases SAW wavelengths, thereby increasing therequired width for a subIDT and limiting the number of subIDTs that canbe incorporated on a reasonable size chip. For example, if the rffrequency is 44.3 MHz and the substrate is Y cut Z propagating lithiumniobate, the SAW wavelength is 77 microns. If 10 periods are reservedfor each code bit, then a maximum of 10 IDTs could be arranged on a 1 cmchip, reserving 1 mm at each end for acoustic absorbing material. If 5of the first code positions are left blank, leaving room for one mainIDT and 4 subIDTs in which to incorporate a code, as proposed byprevious systems, this method permits 2⁴ or 16 possible codes, which isbarely enough for an antitheft system.

(2) The equivalent circuit for a SAW device is a capacitance in parallelwith a resistance. It is well known to those skilled in the art ofelectronics that with a parallel capacitor-inductor-resistor circuit,the maximum response is obtained if the capacitance and inductance arein resonance; that is, the capacitance C and inductance L should satisfythe relation:

    2πf(LC).sup.1/2 =1

where f is the operating frequency. Maintaining this equality for chipscontaining different codes requires that the number of IDTs on each chipbe constant. In a phase-encoding method, this is automatic, since everybit is always "on"; only the phase is altered. In amplitude encoding,some bits, and therefore IDTs, are missing. Each IDT contributes to theoverall SAW device capacitance. Maintaining constant capacitancetherefore further limits the number of available codes. Using theexample given above, a 4-bit code yields 16 distinct codes; a 4-bit codewith the restriction that two bits are 1's and two bits are 0's yieldsonly 6 distinct codes.

This set of restrictions on frequency (below 50 MHz), chip size (lessthan 1 cm chip length), and coding method (amplitude encoding) appearsto lead to an unreasonably low number of distinct codes, rendering thesystem ineffective. The invention disclosed herein breaks this barrierby providing means to reduce spurious signal levels so that the priormethod for reducing spurious signals need not be applied. The inventionrequires that the first two bits be zeroes. This increases the number ofcode bits to 7, yielding 2⁷ or 128 distinct codes. There are 21 distinctcodes with exactly 2 bits "on"; 35 codes with exactly 3 bits "on", etc.These are enough codes for an effective antitheft system. However, thisrequirement necessitates a new method to reduce spurious signals.

METHODS TO REDUCE CROSSTALK

Again referring to FIG. 6, crosstalk originates from SAW energy launchedby one code subIDT (e.g., that shown at reference numeral 52) andreceived by another subIDT (e.g., 56). It is preferred that this type ofprocess yield a much smaller output signal than the same processinvolving the main IDT 50 and one subIDT. One method for accomplishingthis, is physically locating subIDTs in different acoustical channels.²This method is effective but limited. In this method up to eight "taptransducers" or subIDTs may be connected in series with acceptablespurious signal levels. This may be true for phase encoding but it isnot true for amplitude encoding. When amplitude encoding is used andonly the first two bits are zero, only one subIDT can be used in eachparallel acoustic track.

The invention disclosed herein permits the use of two or more subIDTsper acoustic track, and/or allows a designer to dispense with separateacoustic tracks altogether. The invention accomplishes this by fourdistinct techniques:

(1) Time Narrowing

Time narrowing weakens crosstalk signals relative to wanted code signalsby using a narrower electrode width for the subIDTs than is used for themain IDT. The wanted code signals then results from launching andreceiving SAWs with one wide and one narrow IDT; unwanted spuriouscrosstalk signals result from two narrow IDTs and are therefore weaker.

First consider FIG. 7, a standard amplitude-coded delay line with nocompensation for spurious signals. The delay line has a main transducer50 and two subIDTs. SubIDT 52 is positioned to yield an output pulse inbit position 4; subIDT 58, in bit position 7. Ideally, the output wouldconsist of pulses in bit positions 4 and 7. In fact, because ofcrosstalk, there in an additional pulse in bit position 3. Theelectrical pulse output of a device made according to FIG. 7 is shown inFIG. 8. Note that the crosstalk pulse in bit position 3 has the samestrength as the wanted code pulses in bit positions 4 and 7.

Now consider FIG. 9, a time-narrowed coded SAW delay line. Note how incomparison to FIG. 7, the subIDTs 112 and 114, respectively, havenarrower electrode widths compared to the launch IDT 50. FIG. 10presents the electrical pulse output of this device. Note how thisspurious crosstalk pulse in bit position 3 is narrower in time as wellas weakened with respect to the wanted code pulses in bit positions 4and 7. Although this method is illustrated for a simple code with onlytwo code bits "on", it is applicable to codes with more than two bits"on".

(2) Time Shifting

If the antitheft system designer requires a higher immunity to spurioussignals, it is possible to employ a second technique call time shifting.It is well known to those skilled in the art of electronic digitalcommunications that one preferred method to read bit-serial codes is toread the codes at the time corresponding to the expected center or peakof the code-bearing pulses. Spurious signals occurring during thetransition time in between pulses will have a far lower probability ofinterfering with reading the code, than spurious signals occurringduring the center of the code-bearing pulses. It is possible to arrangefor crosstalk spurious signals to occur during these transition times byadding a half code period or bit length X (FIG. 11) to the spacingbetween the main IDT and the subIDTs. If the subIDTs spacing is moperating frequency wavelengths, then the centers of the wantedcode-bearing pulses occur at times mT(n+1/2), where T is the operatingfrequency period and n is any integer.

The spurious crosstalk pulses occur at times mT(i), where i denotes aninteger. This is illustrated in FIG. 12, which presents the electricalpulse output of the device illustrated in FIGURE 11.

In comparing FIGS. 10 and 12, note that in FIG. 10, the spuriouscrosstalk signal occurs at the center of bit 3, while in FIG. 12 itoccurs in between bits 2 and 3, thereby minimizing interference readingbits 2 and 3 correctly as binary zeroes. It has also been found usefulto introduce a 90 phase shift between the spurious crosstalk signals andthe wanted code-bearing signals to ensure that spurious signals are inquadrature with wanted signals. This prevents spurious signals fromdiminishing the amplitude of wanted code-bearing signals, helping toprevent falsely reading a binary one as a zero. While this technique hasbeen illustrated for a two-subIDT code, it is applicable to codes with alarger number of subIDTs.

(3) Time Spreading

Yet another technique to reduce crosstalk is to shape the pattern ofelectrode overlaps within the subIDTs to arrange that the crosstalksignals are spread over a longer period of time and to ensure that theyare weaker than the wanted code-bearing signals. A method to accomplishthis is shown in FIG. 13. SubIDTs 102 and 104 have shaped electrode gappatterns. The shaping is arranged in opposite directions (arrows A andB) for the two subIDTs. This causes the crosstalk pulse arising fromSAWs launched from subIDT 102 and received by subIDT 104 (and viceversa) to be spread out over approximately twice as much time as maincode-bearing pulses launched from main IDT 50 and received by subIDTs102 and 104 (and vice versa).

In addition, the crosstalk signals are weaker. This is illustrated byFIG. 14, which presents the electrical pulse output from a devicefabricated in accordance with FIG. 13. Note that the spurious crosstalksignal in bit position 3 is both spread out in time and weaker than thewanted code signals in bit positions 4 and 7.

The time-spreading principle illustrated in FIG. 13 is applicable onlywhen the number of subIDTs is limited to two, since a third subIDT wouldhave to be shaped like one of the original two subIDTs. The method isstill applicable to more than two subIDTs if it is arranged that thespurious crosstalk occurs at times outside the range of code-bearingpulse arrival times.

(4) Cancellation

A fourth method for reducing spurious crosstalk signals is to addadditional IDTs to the structure to generate additional signals of equalamplitude and time, and opposite phase, relative to the crosstalksignals. These additional signals then cancel the spurious signals. Oneway to arrange this is illustrated in FIG. 15. Careful inspection of theFIGURE will show that main IDT 50 and subIDTs 106 and 108 will producewanted pulses at bit positions 4 and 7 and a crosstalk pulse in bitposition 3 The interaction between additional IDT 110 and main IDT 50produces a cancelling pulse in bit position 3. This cancellation isillustrated in FIG. 16.

There are many ways to produce this cancellation. For example, the mainIDT 50 could have been arranged to be confined to the same acoustictrack width as the subIDTs and the cancellation could have beenaccomplished entirely with additional subIDTs. Acoustic absorbingmaterials well known to those skilled in the art of SAW devices could beapplied to acoustically isolate the additional IDTs from the main codeIDTs and each other.

METHODS TO REDUCE DOUBLE TRANSIT INTERFERENCE

As stated above, double transit interference arises from the reflectionof acoustic energy by an IDT. Methods to reduce double transitinterference rely on reducing this acoustic reflection and employimpedance mis-matching.

It may be shown that, for a convention bidirectional interdigitaltransducer, the difference between reflection loss L₁₁ and transductionloss L₁₃ is

    L.sub.11 -L.sub.13 =10 log.sub.10 (2G.sub.L /G.sub.T)

where G_(T) is the equivalent circuit conductance of the IDT, and G_(L)is the load conductance, which includes both external load conductanceand the conductance of other IDTs³,4. It is desired to arrange for L₁₁to be as large as possible while minimizing increases in L₁₃. Inaccordance with the above equation, this may be accomplished byincreasing the ratio G_(L) /G_(T). There are several ways to do this,including the following two.

(1) Connect an external resistor. This may be done either by directlywiring a resistor across the SAW device or by terminating the pickupcoil with a resistance. Through the transforming action of the twocoils, a resistance connected to either coil will contribute to loadconductance.

(2) Connect additional IDTs to the bus bars, but ensure that they do notinteract acoustically with the code-generating IDTs. This may be doneeither by placement or by covering the added IDTs with acousticabsorber.

REFERENCES

1. Cole, P. H., and Vaughan, R. (1972), "Electronic SurveillanceSystem", U.S. Pat. No. 3,706,094.

2. Skeie, H., (1986), "Surface Acoustic Wave Passive Transponder HavingParallel Acoustic Paths", U.S. Pat. No. 4,620,191.

3. Smith, W. R., Gerard, H. M., Collins, J. H., Reeder, T. M., and Shaw,H. J. (1969), "Analysis of Interdigital Surface Wave Transducers by Useof an Equivalent Circuit Model", IEEE Trans. MTT-17, pp. 856-864.

4. Smith, W. R. (1973), "Minimizing Multiple Transit Echoes in SurfaceWave Devices," Proc. 1973 IEEE Ultrasonic Symposium, pp. 574-577.

Since other modifications and changes varied to fit particular operatingrequirements and environments will be apparent to those skilled in theart, the invention is not considered limited to the example chosen forpurposes of disclosure, and covers all changes and modifications whichdo not constitute departures from the true spirit and scope of thisinvention.

What is claimed is:
 1. A security device that provides a coded surfaceacoustical wave which is converted to an electrical signal forcomparison with a preset code, said security device comprising:asubstrate having means defining an acoustical wave path; and a patternof transducers disposed upon said substrate for providing said codedsurface acoustical wave, including a plurality of spaced-apart taptransducers serially arranged along said acoustical path and operativelyconnected to a main transducer defining a given bit position in saidcode, and wherein a first of said serially arranged tap transducersclosest to said main transducer is disposed a fractional bit lengthfurther from said main transducer than a given length of bits, each bitlength defining a bit position in said code whereby crosstalk isreduced.
 2. The security device of claim 1 wherein said first taptransducer is disposed no closer than a fractional bit length more thanthree bit lengths from said main transducer.
 3. The security device ofclaim 1, wherein an operating frequency is approximately less than fiftyMHz.
 4. The security device of claim 1, wherein an operating frequencyis approximately less than 100 MHz.
 5. The security device of claim 1,wherein at least one tap transducer has an electrode gap pattern ofopposite direction to all of the other tap transducers on saidsubstrate, in order to reduce crosstalk.
 6. The security device of claim1 wherein at least one tap transducer is disposed to produce a signalpulse of opposite phase but approximately equal amplitude with respectto a crosstalk signal pulse in order to cancel same.
 7. The securitydevice of claim 1, further including a transmission coil operativelyconnected across said transducers.
 8. The security device of claim 7,wherein an additional resistance is associated with said coil toincrease load conductance in order to reduce double wave transitinterference.
 9. The security device of claim 1, wherein at least onetap transducer does not acoustically interact with the other taptransducers in order to reduce double wave transit interference.
 10. Thesecurity device of claim 9, wherein the non-interacting tap transducercomprises an acoustic absorber for acoustically isolating saidtransducer.